Top button bar: Add Nested instances

[chrislueders]

As a user I want to change the properties of the components which are inside of the top app bar component, it would be easier to change their properties, if they would be nested instances.

Optional: Add an optional additional icon on the left and right. So that we could show 4 icons on top. Examples are phoenix wallet with 4 icons, relai has 3 icons. It's not often used, but can happen and if we disable it from the beginning, we dont add noise, but we add flexibility IF its needed.